Although I was going to buy an Amiga after seeing it 'released' in Oz at a PC show - I had my first moment of smugness when I first booted up Shadow of the Beast.
There was simply nothing like it graphically on any computer at the time (actually - most arcades at the time too).. So I'd have to lean towards it - cutting edge like the machine itself.
That being said ... Lemmings deserves a definate mention.. although it wasn't as spectacular graphically, it was a highly original game that was birthed on the Amiga.. I'd give it the pride place because it really trod new ground in just about every aspect (including playable with a mouse... that wasn't something you saw every day either).
Beast, and many other games, at the end of the day (although stunningly presented) were ideas we'd seen before...